R help - Jul 2009 - Aggregate, max and time of max

All,

For data consisting of serial measurements on subjects, one may use the
aggregate function to say compute the peak response for each subject for
each design condition.  Is there a way to alter this or another one-liner to
also retain the time at which the peak occurred and thus avoid writing a
doing this via a loop?  I suppose one could attempt to employ the split
function but that's probably no simpler than employing a loop.  Sample code
below:


data = expand.grid(time = seq(1,6), subject = seq(1,20), treatment
c("placebo", "drug"))
data.ex = cbind(y = rnorm(dim(data)[1], 5, 1), data.ex)

data.peak = aggregate(data.ex[c(1)], data.ex[c(3,4)], max)
## this provides the peak of each subject on each treatment, but time is
## lost.  Including time in the statement doesn't help clearly as then
## the peak of all the times will be calculated


David

--------------------------------------
David Afshartous, Ph.D.
Research Assistant Professor
University of Miami, Miller School of Medicine
Division of Clinical Pharmacology
1500 N.W. 12th Avenue, 15th Floor West
Miami, Florida 33136

E-mail: afshar at med.miami.edu
Phone: +1 305-243-1549

Hello,

I believe that

by( data.ex, data.ex[,c(3,4)], function(x) x[which.max(x[,1]),] )

does what you want. Then,

do.call( rbind, by( data.ex, data.ex[,c(3,4)], function(x)
x[which.max(x[,1]),] ) )

looks somewhat nicer.

Best regards,

Carlos J. Gil Bellosta
http://www.datanalytics.com


On Fri, 2009-07-24 at 13:06 -0400, Afshartous, David
wrote:
> All,
> 
> For data consisting of serial measurements on subjects, one may use the
> aggregate function to say compute the peak response for each subject for
> each design condition.  Is there a way to alter this or another one-liner
to
> also retain the time at which the peak occurred and thus avoid writing a
> doing this via a loop?  I suppose one could attempt to employ the split
> function but that's probably no simpler than employing a loop.  Sample
code
> below:
> 
> 
> data = expand.grid(time = seq(1,6), subject = seq(1,20), treatment >
c("placebo", "drug"))
> data.ex = cbind(y = rnorm(dim(data)[1], 5, 1), data.ex)
> 
> data.peak = aggregate(data.ex[c(1)], data.ex[c(3,4)], max)
> ## this provides the peak of each subject on each treatment, but time is
> ## lost.  Including time in the statement doesn't help clearly as then
> ## the peak of all the times will be calculated
> 
> 
> David
> 
> --------------------------------------
> David Afshartous, Ph.D.
> Research Assistant Professor
> University of Miami, Miller School of Medicine
> Division of Clinical Pharmacology
> 1500 N.W. 12th Avenue, 15th Floor West
> Miami, Florida 33136
> 
> E-mail: afshar at med.miami.edu
> Phone: +1 305-243-1549
> 
> ______________________________________________
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